This invention relates to friction stir welding and riveting, more particularly, to methods of joining multiple workpieces using a stir rivet to create a mechanical weld, an interweld, and a diffusion bond.
Friction stir welding (FSW) is a method used to join metal workpieces. The method generally uses a cylindrical, shouldered tool with a profiled pin that is rotated at the joint line between two workpieces while being traversed along the joint line. The rotary motion of the tool generates frictional heat which serves to soften and plasticize the workpieces. This softened material, contributed by both workpieces, intermingles and is consolidated by the pin shoulder. As the pin moves laterally the frictional heating is reduced and the softened material hardens, creating a bond between the two workpieces. The best current understanding of the process is that no melting occurs and the weld is left in a fine-grained, hot worked condition with no entrapped oxides or gas porosity.
A common design of FSW stir rods is that the stirring element is substantially symmetrical with some irregularity to induce a stirring motion. Frequently the stir rod has a threaded appearance similar to a bolt. However, to promote intermingling and to retain the plasticized material in the weld zone for as long as possible the direction of rotation of the rod is such that the threads carry the plasticized material downward to create as turbulent a flow and as efficient an intermingling as possible. Particularly for metal workpieces the high thermal conductivity strongly localizes the region which is plastic enough to be deformed by the stirring action. Thus, the width of the stirred region is substantially equal to the width of the stirring rod.
This invention is based on a newly developed method which we call friction stir riveting. This method improves friction stir welding by using a stir rivet having a slideable cap and an angled stir member. The stir rivet is rotated and advanced into a pair of workpieces to plasticize material around the rivet for stir welding the workpieces together. Near the beginning of the process, the slideable cap contacts the first workpiece. The contact between the cap and the first workpiece creates a partial seal limiting the amount of plasticized material displaced from the stir site. As the rivet further progresses into the workpieces, a pivot member extends away from the body of the rivet to increase the stir radius around the rivet. The rivet is then left in place to form a weld between the rivet and the solidified material.
The present invention utilizes a friction stir rivet having an elongated body including a cylindrical section and enlarged upper and lower stops at opposite ends of the cylindrical section. The cylindrical section of the body extends through a slideable cap, which rests above the lower stop. A slot extends longitudinally along the cylindrical section and houses a portion of an angled pivot stir member. An interlocking guide extends longitudinally along a portion of the cylindrical section. Preferably, the interlocking guide on the cylindrical section is a flat surface.
The cap has a central opening surrounding the cylindrical section. The central opening of the cap has an interlocking guide mateable with the interlocking guide of the cylindrical section, which causes the cap to rotate with the body. Preferably, the interlocking guide in the central opening of the cap is a flat surface.
The upper stop forms the head of the rivet and provides an upper boundary, limiting upward travel of the cap. The lower stop serves as a lower boundary limiting downward travel of the cap. The underside of the lower stop forms the lower end of the rivet, which is the first portion of the rivet to frictionally contact the workpieces to be joined.
A recessed socket is centrally located on the upper portion of the upper stop and is aligned with the rotational axis of the rivet. To rotate the rivet, a rotational apparatus is inserted into the recessed socket.
The rivet, when rotated, locally softens and penetrates the bodies of the workpieces, creating a cavity filled with plasticized material. Shortly after the lower end of the rivet penetrates the first workpiece, the slideable cap contacts the first workpiece to create a seal around the stir site, thereby limiting the amount of displaced plasticized material seeping out of the cavity.
As the rivet advances into the workpieces, the cap slides up the elongated section of the rivet, engaging the actuating portion of the stir member and causing the actuating portion to pivot into the slot. As the actuating portion of the stir member pivots into the slot, the stirring portion of the member pivots outward into the workpieces, increasing the stir radius around the rivet. Upon reaching a desired depth the rotating motion is stopped and the joint is cooled to provide an internally welded joint maintained together partially by the mechanical shape of the rivet and partially by the welding of the components together.
The elongated section of the rivet has a smaller radial thickness than the lower stop, to create a re-entrant portion around the cylindrical section. When the rivet is embedded in the workpieces, the re-entrant portion between the cap and the lower end fills with plasticized material, increasing the volume of retention around the rivet. Allowing plasticized material to fill the re-entrant portion around the rivet increases the strength of mechanical retention.
During the process, the slideable cap restricts oxygen access around the rivet by creating a seal between the rivet and the first workpiece. The reduced oxygen supply around the rivet reduces the formation of oxidation on the cylindrical section of the rivet which provides a clean surface to form a bond with the plasticized material. Allowing formation of an oxide layer would interfere with bonding between the cylindrical section and the plasticized material.
The rivet should be formed of a relatively high melting point metal or refractory metal so that the rivet has a higher melting point than the workpieces to be joined. Preferably, the rivet should have a melting point that is at least 100xc2x0 Fahrenheit higher and more preferably at least 200xc2x0 Fahrenheit higher than workpieces, such as aluminum. Further, the rivet should be formed of a metal of substantially greater hardness than the metal workpieces to be joined. Exemplary metals include high carbon steel, titanium (e.g. titanium 6-4) and the like. Preferably, the rivet should be formed of a metal that is capable of forming a diffusion bond with the metal workpieces to be joined.
A rotational apparatus is used to rotate and press the rivet into the metal workpieces to be joined. The rivet penetrates best when it is rotated at speeds between 4,500 and 27,000 revolutions per minute. The amount of pressure needed to allow the rivet to penetrate the metal workpiece depends upon the speed of rotation. The rate of penetration is increased when the amount of pressure applied is increased, or when the revolutions per minute are increased. Under good conditions, a friction stir rivet can penetrate aluminum at up to 27 millimeters per minute.
The foregoing description is directed, as an example, to joining aluminum metal workpieces with a stir rivet made of metal with a higher temperature melting point. However, it should be understood that other fusible materials may be joined using the same process with a proper selection of compatible materials. Thus, other metals and thermoplastics may also be successfully joined with a stirring rivet and process within the guidelines above described.